Processes favored in the United States for the production of soda ash are the sodium carbonate sesquicarbonate process and the sodium carbonate monohydrate process. Both processes purify crude trona to produce refined soda ash. Almost all of the soda ash produced in the United States is obtained from a vast deposit of crude, mineral trona (Na2CO3.NaHCO3.2H2O) located in Green River, Wyo. The trona deposit is made up of several trona beds about 800 to 3000 feet underground. These trona beds are separated by layers of shale that generally overlap each other. The quality of the trona varies depending upon its location in the deposit. Crude trona consists primarily of about 87-88% of sodium sesquicarbonate (Na2CO3.NaHCO3.2H2O) and about 11-13% insoluble clays and shales, and in lesser amounts, sodium chloride (NaCl), sodium sulfate (Na2SO4), and organic matter. The amount of impurites present is sufficiently large so that this crude trona must be purified to remove or reduce the impurities before the soda ash or other sodium carbonate salts derived from the trona can be produced and sold for commercial use.
Sesquicarbonate
The Sesquicarbonate processing steps involve: dissolving the crude mined trona in a cycling, hot mother liquor containing excess normal carbonate over bicarbonate in order to dissolve the trona congruently, clarifying the insoluble muds from the solution, filtering the solution, passing the filtrate to a series of vacuum crystallizers where water is evaporated and sodium sesquicarbonate crystals form and are separated from the mother liquor, recycling the mother liquor to dissolve more crude trona; and calcining the sesquicarbonate crystals to soda ash.
Monohydrate
The Monohydrate processing steps involve: calcining the crude mined trona converting it to crude soda ash, dissolving the crude soda ash in water, clarifying the resulting sodium carbonate liquor to remove insoluble muds from the solution, filtering the solution, passing the filtrate to an evaporator circuit (multiple effect evaporator or mechanical vapor recompression evaporator) where water is evaporated and sodium monohydrate crystals are formed and separated from the mother liquor, recycling the mother liquor to the evaporator circuit, and calcining the monohydrate crystals to soda ash.
The processes for the production of soda ash and other sodium carbonate salts employ crystallization steps that concentrate impurities in the mother liquors. Purge streams are required in these processes (Solvay, Sesquicarbonate, Monohydrate, Decahydrate, Sodium Bicarbonate, and processes that recover sodium values from solution mining liquors) to control impurity levels to meet quality requirements. Processes have been described in the prior art for recovering the alkali values from these various waste streams. Processes that employ the sodium decahydrate process must eliminate or significantly reduce the sodium bicarbonate concentration through decarbonizing using steam stripping, sequential crystallization, or addition of expensive neutralizing agents such as caustic soda or lime. Processes that produce more than one product are limited by the demand for the less widely used product: sodium sesquicarbonate, light or medium density soda ash, or sodium bicarbonate; otherwise, the product in less demand might be further processed to dense soda ash. These prior art methods are typically complex procedures, involving multiple steps in which various forms of sodium carbonate are crystallized, and these multiple crystallization operations add significantly to the overall economic cost of the soda ash recovery processes.
Typically, in the manufacture of soda ash, a system of storage ponds has been used to accommodate disposal of the insoluble grits and muds, purge streams, mine water, and other sources of waste waters inherent to the process. These ponds are also used to maintain water balance through the process of natural evaporation and can provide a source of cooling water for the evaporator trains. Over the years, a significant amount of the total mined sodium carbonate has been lost from processing as a constituent in the water discharge. As this sodium carbonate-bearing water is discharged to the ponds, the water evaporates, leading to continual concentration of the soda ash in the ponds. This soda ash resides in the system through the deposition of sodium carbonate decahydrate crystals on the pond bottoms, reducing the total pond volume. A substantial amount of sodium carbonate decahydrate, which in its crystalline form naturally excludes impurities, resides in these ponds. This reduction in volume results in two major problems for a facility of substantial commercial size; first, the need to expand existing or constructing additional waste cells to impound insolubles; and second, inadequate evaporation and hence inadequate cooling of the water used for cooling purposes.
Although the insolubles amount to only a small faction of the ore, over time these accumulate to sizeable volumes, for example, amounting to over 288,000 tons per year when operating a plant producing 2,400,000 tons per year of soda ash. Several methods for disposing of these grits, muds, and waste streams have been described in the prior art and involve a process of returning these insolubles to the mine taking advantage of the space created in mined-out areas. However, many problems exist because of the presence of water associated with the solid impurities. The water will drain from the solids over time, creating a messy and hazardous condition that must be confined and the water collected. For example, the water has been known to dissolve the ribs and floor in areas where subsidence has not occurred, compromising the mining panel. Recovery of water in subsided panels presents other problems. To improve recovery efficiency, the addition of alkaline hydroxides or alkaline earth metals, to concentrations up to 10% of the aqueous slurry solution are described has been employed.
A complicating factor in dissolving trona deposits is that sodium carbonate and sodium bicarbonate that comprise the trona have different solubilities and dissolving rates in water. These incongruent solubilities of sodium carbonate and sodium bicarbonate tend to cause bicarbonate “blinding” when employing solution mining techniques. “Blinding” is an occurrence which has long been recognized as a problem by the art in the solution mining industry.
Solution mining techniques suffer from several disadvantages. It is apparent that a significant continuing problem associated with solution mining is the subsequent recovery of sodium carbonate from relatively low concentration of carbonate and bicarbonate in the solution brine. Unless the bicarbonate concentration is reduced, solution mining brines will contain an unacceptable high level of sodium bicarbonate and other impurities to prevent processing into sodium carbonate by the conventional monohydrate process. A major problem experienced is due to the co-precipitation of sodium sesquicarbonate crystal during the sodium carbonate monohydrate crystallization which reduces the quality of the final product. Another difficulty with underground solution mining is that the requirement of the high temperatures that are needed to increase the dissolving rate of trona and yield highly concentrated solutions required as feed to a conventional sodium carbonate monohydrate process. Substantial energy is required to heat the solvent sufficiently to off-set heat losses to the earth. Processes that add alkalis such as sodium hydroxide or lime to the solvent for solution mining reducing the energy requirements while increasing the dissolving rate have been demonstrated, but these neutralizing agents are unfortunately expensive.
Methods to process such sodium bicarbonate-bearing solutions have also been described and comprise a conversion of the alkali values to a more desirable crystal or crystals that can then be separated and processed to soda ash. When the processes involve two crystallization steps such processes yield two different species of sodium carbonate salts. Such processes describe the addition of expensive neutralizing agents to convert the sodium bicarbonate to sodium carbonate, steam or air stripping to convert the bicarbonate to carbonate and carbon dioxide gas, and even enriching the solution by contact with conventionally mined trona or carbonating the solution with carbon dioxide gas.
The economic attraction of solution mining is to avoid such costs as sinking new mine shafts, employing miners and equipment underground, and supporting mechanical mining operations. But, these benefits are offset by the cost of adding neutralizing agents to the slurries pumped into the mine and the operational costs and limitations associated with producing soda ash from such recovered streams.
A process as herein detailed that reduces the volume of the waste streams reporting to evaporation ponds would prolong the life of the ponds and allow a more economic option for grits and mud disposal than would be experienced employing underground disposal methods. Water balance would be achieved with the benefit of alkali recovery of the decahydrate deposits. A process of this kind would enrich the weak sodium carbonate-bearing streams, and if desired, precipitate sodium bicarbonate from such streams to produce a dilute sodium bicarbonate solution rich in sodium carbonate and suitable for recovering soda ash employing sodium decahydrate or sodium monohydrate processes. Such a process would also allow selective precipitation of other sodium carbonate salts such as sodium sesquicarbonate, sodium bicarbonate, and various densities of soda ash.